The Irishman, 42, will have a hearing before French racing authorities next month and could face a lengthy ban.

Fallon tested positive in August for a banned substance after Myboycharlie's win at the Prix Morny in France.

"I had a letter at the weekend that confirmed the positive findings," his lawyer Christopher Stewart-Moore told the Times.

"The letter tells me that the Medical Commission wish to meet in January to consider the case.

"I have instructed our French legal counterparts to request all the relevant data before we decide on our next step."

Stewart-Moore has previously said some aspects of the test result were "curious".

The news that Fallon had tested positive for a banned substance, believed to be cocaine, emerged the day after his race-fixing trial collapsed.

The six-time champion jockey was acquitted of conspiracy to defraud at the Old Bailey on 7 December.

If convicted of a second offence in such a short space of time the authorities will take a very dim view of it

5 Live racing correspondent Cornelius Lysaght

Fallon was also given a six-month ban by France Galop, the governors of racing in France, in December 2006 after testing positive for a metabolite [produced by the body after it has processed a chemical] of a prohibited substance five months earlier at Chantilly.

The suspension effectively meant he could not ride on any racecourse in the world, as other governing bodies have a reciprocal suspension policy in such cases.

He could face another lengthy spell on the sidelines, with previous similar cases earning suspensions of a year to 18 months.

"If convicted of a second offence in such a short space of time the authorities will take a very dim view of it," said BBC Radio 5 Live racing correspondent Cornelius Lysaght.